The ongoing miniaturization of feature sizes in semiconductor manufacturing processes has facilitated the formation of microscopic structures, i.e. structures that have feature sizes in the micron and submicron, e.g. nanometer domain, on substrates such as silicon substrates. A prime example of such a microscopic structure is a microelectromechanical system (MEMS) structure. Such structures are sometimes also referred to as micromachines.
MEMS structures can be used for a wide range of applications in different fields of technology, e.g. electronics, medicine, pharmacy and chemistry. Applications in the filed of electronics for instance include accelerometers, gyroscopes, sensors, and so on. The MEMS structures may be made from any suitable material, e.g. silicon, polymer, metals amongst others.
Typically, the MEMS structure requires a certain degree of translational freedom in order to perform its function. To this end, the MEMS structure is packaged such that the structure is located in a cavity.
In the paper “Wafer Level Encapsulation Technology for MEMS Devices using a HF-Permeable PECVD SIOC Capping Layer” by G. J. A. M. Verheijden et al. in MEMS 2008, IEEE 21st conference on Micro Electro Mechanical Systems 2008, pages 798-801, a novel technology for the encapsulation of MEMS devices using a porous capping material is disclosed. The capping material consists of a low temperature PECVD layer of SiOC and is shown to be permeable to HF vapor and H2O and therefore allows for the removal of a SiO2 sacrificial layer and the formation of a cavity underneath the capping layer, with the capping material being permeable enough to allow for the evacuation of the SiO2 reaction products. The cavity underneath the capping layer allows for high-Q operation of a MEMS resonator. A sealing layer can be deposited on the capping layer without significantly contaminating the cavity. However, it has been found that residual contamination of the cavity is difficult to avoid, especially when a sealing layer consisting of relatively small molecules is formed.
US patent application No. 2006/0108652 A1 discloses a technique for manufacturing a MEMS device similar to the technology disclosed in the aforementioned paper, in which the residual contamination of the cavity upon deposition of the sealing layer is acknowledged. This contamination may be avoided by densifying the capping layer through an anneal step. However, such a step may cause damage to parts of the device that are intolerant to elevated temperatures.